Nonaqueous secondary batteries that produce electromotive force by doping/dedoping of lithium (lithium ion secondary batteries) are characterized by high energy density and are therefore widely used as main power supplies for portable electronic devices such as cellular phones, laptop computers and the like. In light of demands for higher performance and longer operation of such portable electronic devices, active research and development is being carried out with the aim of increasing energy density and output. Upsizing is another target of investigation for use as power sources in automobiles and the like. When dealing with increases in energy density, output and size, however, one of the major issues to be considered is safety assurance.
Lithium ion secondary battery separators currently employ polyolefin microporous membranes composed mainly of polyethylene, and such batteries are provided with a shutdown function to ensure safety. This function is described in Japanese Patent Publication No. 2642206. The shutdown function is a function whereby the polyolefin constituent material of the separator melts to close the pores and markedly increase the resistance of the separator. When a problem occurs in the battery that causes temperature rise, the internal resistance of the battery increases by this function to essentially stop the flow of current and ensure safety.
Because the shutdown function is based on the working principle of shutting the pores by melting of the constituent material, exposure of the battery to even higher temperature causes rupture of the separator (meltdown) and internal shorting between the positive electrode and negative electrode, rendering the battery extremely dangerous. Increasing the energy density, output and size of a lithium ion secondary battery causes a faster heat release rate when a problem occurs, and therefore closing of the pores is not sufficiently rapid and shutdown fails to function, thereby increasing the risk of meltdown. As a result, it has been difficult to ensure safety with conventional separators having a shutdown function when the energy density, output and size are increased, such that highly heat resistant materials that do not undergo meltdown have been essential.
A separator that is obtained by laminating a polyethylene microporous membrane and a polytetrafluoroethylene microporous membrane has been proposed in J. Electrochem. Soc., 140, L51(1993), for both a shutdown function and heat resistance that prevents meltdown. The shutdown function is satisfactorily exhibited, and no meltdown has been observed in temperature ranges of up to 250° C. Examples of laminating porous membranes composed of polyolefin microporous membranes and heat resistant resins are proposed in Japanese Unexamined Patent Publication HEI No. 10-3898, Japanese Unexamined Patent Publication No. 2002-25526 and Japanese Unexamined Patent Publication No. 2003-123724.
In addition, composite porous membranes obtained by integrally coating porous layers made of heat resistant resins onto polyolefin microporous membranes have been proposed in Japanese Unexamined Patent Publication No. 2001-23600 and Japanese Unexamined Patent Publication No. 2002-355938, as separators with both a shutdown property and heat resistance.